Apple Watch 7 backcover came off easily now it won't turn on again

The back of my Applewatch 7 came off easily without me doing anything. Unfortunately, this was only noticed when I was in the water with it and the watch suddenly started flashing strangely. now it's not turning on again. probably because of the water inside the watch...

I contacted Apple Support, but they only told me that my warranty had expired (device purchased directly from the Apple Store on April 22, 2022) and that the repair would cost around €370.

I have to say I'm very disappointed because I would have expected better quality. Well maybe it's their strategy to purchase apple care. who knows.

Does anyone have any idea whether such a water damage repair can be carried out yourself and how much it would cost?

[Re-Titled by Moderator]